Method of treating ammonia-comprising waste water

ABSTRACT

The invention relates to a method of treating ammonia-comprising waste water in which the bicarbonate ion is the counter ion of the ammonium ion present in the waste water. According to the invention half the ammonium is converted into nitrite, yielding an ammonia and nitrite-containing solution, and in the second step the nitrite is used as oxidant for the ammonia. In the method according to the invention the conversion of half the ammonia into nitrite occurs automatically, providing a method which requires fewer controls. Also, the method according to the invention requires no external additive.

The present invention relates to a method of treating ammonia-comprisingwaste water, wherein in a first step ammonia-comprising waste water issubjected to a nitrification reaction treatment by using a nitrifyingmicroorganism and by the addition of oxygen, yielding a solutioncomprising an oxidation product of ammonia, and in a second step theoxidation product of ammonia together with ammonia is converted intonitrogen, through the influence of a denitrifying microorganism.

Such a method is known from U.S. Pat. No. 5,078,884. The nitrate formedby the oxidation of ammonia is used as an oxidant to convert, undersubstantially anaerobic conditions and with the aid of a microorganism,still to be degraded ammonia which acts as electron donor. During thisprocess nitrogen is formed which is discharged into the atmosphere.

The disadvantage of this method is that there is no reliable degradationof ammonia, as can be seen in (the right half of) FIG. 2 of saidpublication. This means that undesirable discharges into the surfacewater may take place. To prevent this, all kinds of investments such ascontrol and measuring equipment would be required. Finally, this methodinvolves the addition of lye (see reaction equation 5 of saidpublication), which must also be controlled.

The objective of the present invention is to improve the methodaccording to the preamble and in particular to provide a cost-effective,more reliable method which requires no additives and partly because ofthat, is simpler from a control-technological point of view.

Surprisingly, this objective can be achieved by a method which ischaracterized in that by using bicarbonate-containing waste water whichis substantially stripped of bicarbonate by the supply of air, and inthe first step maintaining the pH at ≦7.2 by controlling the aeration,part of the ammonia present in the waste water is converted intonitrite, yielding a nitrite-containing solution, and in the second stepthe denitrifying microorganism uses the thus formed nitrite as oxidantfor the remaining ammonia.

In this manner a method is provided offering considerable advantages,one of which being a substantially more self-regulatory nature.Moreover, the use of additives is avoided.

From the prior art a method is known, comprising a nitritification stepin which ammonia is converted into nitrite. In a denitritification stepthe nitrite is converted into nitrogen while an organic carbon source isadded as substrate for a denitritifying organism. As organic carbonsource methanol is used. When said methanol is degraded, acid, formedduring the nitritification step, is consumed, providing the pH controlof the denitritification process. The disadvantage of thisnon-continuous method is that an additive is required and much controlis required such as time control and substrate feed control. The totalconversion of ammonia is not under all conditions satisfactory and islimited to at the most 90% and for this reason a subsequent treatment isoften required.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows relevant parameters of the invention.

FIG. 2 is a chemical formula sheet according to the invention.

A first favourable embodiment of the method is characterized in that inthe first step the ammonia-containing waste water is fed to anitritification reactor in an amount such that the nitritificationreactor operates without sludge retention, the temperature of a solutionsubjected to nitritification is maintained at between 25 and 60° C. andthe pH at between 6 and 7.2.

In this manner favourable conditions are provided for the conversion ofammonia to nitrite, not nitrate.

In the second step, the temperature of the solution in thedenitritification reactor being subjected to the denitritificationtreatment is preferably maintained at between 25 and 60° C. and the pHat between 6 and 9.

In this manner favourable conditions are provided for the conversion ofnitrite and ammonia into nitrogen.

According to a second embodiment of the method in accordance with theinvention the first step and the second step are carried outsimultaneously in one bioreactor, in which bioreactor nitrifying anddenitrifying microorganisms are present in a solid phase, the nitrifyingmicroorganism being substantially present in the outer, aerobic part ofthe solid phase and the denitrifying microorganism being substantiallypresent in the anaerobic inner part of the solid phase, and oxygen,dependent on the ammonia concentration in the bioreactor, is supplied inan amount limiting the first step.

The advantage of this method is that only one reactor is required. Thepreferably somewhat rough carrier comprises a biofilm, that is to say alayer comprising the nitrifying and denitrifying microorganisms, and thefirst step takes place in the outer, aerobic part of the biofilm.Because of oxygen limitation, this outer, aerobic part of the biofilm isunable to convert more than 50% of the ammonia supplied and the ammonia,together with the nitrite formed in the outer, aerobic part of thebiofilm, will be converted in the inner, aerobic part of the biofilminto nitrogen. The solid phase may consist of a spontaneously formedaggregate.

Preferably as solid phase a phase chosen from a biofilm-carryingparticulate carrier and a biofilm-carrying immobile carrier is used.

If the carrier is particulate, the size of the carrier is preferably 0.1to 1 mm. In this manner a high biomass density is provided and the sizeof the bioreactor can be kept to a limit. In a typical case the ammoniaload of the biofilm surface is greater than 2-33 g N/m²·day. Someroughness of the carrier surface is an advantage.

In the second step the temperature of a solution subjected to thedenitritification-nitritification reaction in the reactor is preferablymaintained at between and 60° C. and the pH at between 6 and 9.

This creates favourable conditions for the conversion of ammonia intonitrogen.

The invention will be further elucidated by means of the detailedspecification hereinbelow referring to an

Example of the method according to the invention, the parameters ofwhich are given in the accompanying FIG. 1.

The method according to the present invention comprises anitritification reaction and a denitritification reaction. As can beseen from the overall-reaction equation I shown on the formula sheetwhich is FIG. 2, and as will be elucidated below, no addition ofpH-adjusting means is necessary. The nitritification according toreaction equation II yields two equivalents of protons per convertedequivalent ammonia. In waste water that can be suitably treated byapplying the invention, HCO₃ ⁻ is the counter ion of the ammonia ion.Examples of such waste water include the percolation water from rubbishdumps and the effluent from anaerobic purifications. The supply of airnecessary for the oxidation of ammonia during the nitritificationreaction, also ensures that CO₂ is removed in accordance with reactionequation III. Accordingly, the nitritification reaction has a net yieldof one equivalent acid per converted equivalent ammonia. Thedenitritification reaction according to reaction equation IV requiresone equivalent acid per converted equivalent ammonia. This means that byusing the CO₂ stripped effluent from the nitritification reactor, no pHadjustment is required.

By removing CO₂, the solution subjected to nitritification is strippedof HCO₃ ⁻, lowering the buffering capacity of the solution. This meansthat its pH may vary, in particular that it may drop due to theformation of acid in this step. The pK of HCO₃ ⁻, that is to say the pHat which HCO₃ ⁻ buffers optimally, is 6.37. The drop of the pH inhibitsthe nitritification process, and ammonia is converted only partly but,according to the understanding provided by the present invention, to thedesired extent (namely up to 50%). As a result, a suitable amount ofammonia is available for the subsequent denitritification process whichuses acid and thus benefits from the low pH.

Without being bound to any theory, applicant assumes that the goodcontrolling quality is achieved due to the fact that the nitritifyingorganism only perceives NH₃ and HNO₂ and not NH₄ ⁺ and NO₂ ⁻. When thepH drops the NH₃ (substrate) concentration decreases and the HNO₂concentration which inhibits the nitritification reaction, increases.

EXAMPLE

An agitated batch reactor (2.4 l) was operated without sludge retentionand fed with ammonia-rich waste water (41 mM; pH ≅8.0). During the day80% of the reactor volume was replenished. The temperature wasmaintained at 33° C. and the dissolved oxygen concentration was 20%.Under these conditions the biomass concentration was 140 mg dry weightper litre. The pH of the reactor's effluent was about 6.7. N.B.: the pHwas not adjusted by the addition of a substrate compound such asmethanol. 40-50% of the ammonia in the waste water was converted.

1.2 ml/min of the effluent was fed into a fluidized-bed reactor(capacity 2 l). The pH in the fluidized-bed reactor was stable and wasabout 7.9. Kjeldahl nitrogen was converted in the fluidized-bed reactorat a rate of 0.6 kg N/m³·day. The total removal of nitrogen was 83%. Therelevant parameters are given in the Figure.

This percentage may be increased by returning a portion of the effluentfrom the fluidized-bed reactor to the batch reactor. Due to the stablepH in the fluidized-bed reactor the amount of return is not verycritical and may be set at a permanent value. Too high a return resultsin an increased degradation of ammonia in the batch reactor. This causesthe consumption of acid to drop in the fluidized-bed reactor, as aconsequence of which the degradation of ammonia decreases again.

Although the method according to the invention has a substantially moreself-regulatory nature and automatically results in an overalldegradation of ammonia of at least about 80%, the pH of the contents ofthe nitritification reactor may optionally be adjusted by means of thesupply of effluent from the denitritification reactor. Thus here noexternal additive whatsoever is added.

According to another embodiment of the method according to theinvention, the waste water is fed to a denitritification step. Duringdenitritification ammonia from the waste stream is used and a portionfrom the denitritification reactor is fed to the nitritificationreactor. There nitrite is formed which is fed to the denitritificationreactor.

In the event that the effluent from the denitritification step isdischarged, said effluent may first be subjected to a nitrification stepin order to avoid nitrite discharge.

Suitable microorganisms may be obtained without great difficulty fromsludge of existing water-treatment plants in which ammonia is degraded,in the manner described in the literature. Alternatively, the culturefor the denitritification may be obtained from the Central Bureau voorSchimmelcultures, Baarn, the Netherlands, registered under number949.87.

What is claimed is:
 1. An improved method of treating ammonia-comprisingwaste water, wherein in a first step ammonia-comprising waste water issubjected to a nitrification treatment by using a nitrifyingmicroorganism and by addition of oxygen, yielding a solution comprisingan oxidation product of ammonia, and in a second step the oxidationproduct of ammonia together with ammonia is converted into nitrogen,through influence of a denitrifying microorganism, wherein theimprovement comprises using bicarbonate-containing waste water which issubstantially stripped of bicarbonate by supplying air, and in the firststep said ammonia-comprising waste water is fed to a nitrificationreactor in an amount such that the nitrification reactor operateswithout sludge retention and pH is maintained at less than or equal toapproximately 7.2 by controlling the aeration, thereby automaticallylimiting conversion of the ammonia present in the waste water intonitrite as the oxidation product of ammonia to up to fifty percent ofsaid ammonia and substantially preventing conversion of said ammonia tonitrate, yielding a both ammonia and nitrite-containing solution, and inthe second step the denitrifying microorganism uses the thus formednitrite as oxidant for the remaining ammonia.
 2. A method according toclaim 1 wherein in the first step temperature of a solution subjected tonitrification is maintained at between approximately 25° and 60° C. andpH is between approximately 6 and 7.2.
 3. A method according to claim 1wherein in the second step, the temperature of the solution in thedenitrificaton reactor being subjected to the denitrification treatmentis maintained at between approximately 25° and 60° C. and the pH atbetween approximately 6 and
 9. 4. A method according to claim 1 whereina temperature of a solution in the reactor being subjected to adenitrification-nitrification treatment is maintained at betweenapproximately 5° and 60° and the pH at between approximately 6 and 9.